Via naturalistic observations, parent interview, and direct assessments, we examined language proficiency, language use, and differentiation of a 3-year, 4-month-old bilingual child exposed to Mandarin and English via the "one parent-one language" principle. Although noun versus verb dominance has been explored across verb-based (Mandarin) and noun-based (English) languages in a between-subjects manner with monolinguals, it has not been explored within bilingual children learning both languages from birth. Three 15-minute sessions were recorded in the home: child-mother Mandarin interaction, child-father English interaction, and two parent-child bilingual interactions. The child was dominant in Mandarin according to number of words used, mean length of utterance, and receptive vocabulary. The child used the two languages discriminately depending on the interactive contexts. However, the languages used by all three members of the family contained more language mixing than was perceived by the mother during the interview. About 5% of the mother and child speech involved language mixing (use of the nontarget language), and the rate of nontarget language use in child-father interaction was 9%. Although maternal and paternal language input to the child were similar in terms of noun/verb usage, the child used proportionately more verbs than nouns during child-mother Mandarin interaction and used more nouns than verbs in child-father English interaction.